The present disclosure relates to process devices used in process control and monitoring systems. More specifically, the present disclosure relates to performing loop current diagnostics to identify errors in a process device.
Process variable transmitters are used to measure process parameters (or process variables) in a process control or monitoring system, and transmit the measurement to a control room. For example, information related to pressure of process fluid may be transmitted to a control room and used to control the process, such as oil refining.
The transmission of the process variable measurement is often performed over a two-wire process control loop. In such a process control loop, current is supplied from a current source at a location such as in the control room, and the transmitter controls the current flowing in the loop from its remote location in the field. For example, a 4 mA signal can be used to indicate a zero reading and a 20 mA signal can be used to indicate a full scale reading. The same two wires can also be used to power the transmitter.
More recently, transmitters have employed digital circuitry which communicates with the control room using a digital signal which is superimposed onto the analog current signal flowing through the process control loop. One example of such a technique is the HART® communication protocol. The HART® protocol, and other such protocols typically, include a set of commands or instructions which can be sent to the transmitter to elicit a desired response, such as transmitter control or interrogation.
When an analog loop current is used to represent information such as a process variable, the accuracy at which the loop current can be set can be a limiting factor on the accuracy of the transmitted process variable. As the transmitter ages, the ability of the transmitter to control the loop current to accurately represent the process variable measurement can degrade. Accordingly, it is desirable to periodically perform a diagnostic test on the transmitter by checking that the loop current is an accurate representation of the process variable measurement. Exemplary devices that perform this diagnostic function are described in U.S. Pat. No. 7,280,048 and U.S. Publication 2005/0030186, which are assigned to Rosemount Inc. of Chanhassen, Minn.
The diagnostic tests performed by a transmitter need to be as accurate as possible in order to prevent falsely identifying a transmitter as failing. Conventional transmitter diagnostic tests may falsely identify a transmitter as failing during periods when the processes variable is changing. The changing process variable can result in a transmitter setting a loop current to a value that differs from the process variable measurement even though the transmitter is operating properly.